Fialuridine (FIAU) is a nucleoside analog with antiviral activity. It inhibits thymidine kinases from herpes simplex virus types 1 and 2 with K values of 0.14 and 0.95 µM, respectively, while blocking green monkey Vero cell thymidine kinase less effectively (K = 53 µM). FIAU blocks DNA synthesis in human cytomegalovirus and hepatitis B, as well as herpes simplex. Notably, while FIAU has few adverse effects in several mammals, it causes mitochondrial toxicity leading to liver damage and death in humans.